Plasma processing systems are routinely used to modify the surface properties of substrates used in applications relating to integrated circuits, electronic packages, and printed circuit boards. In particular, plasma processing systems are used to treat surfaces in electronics packaging, for example, to increase surface activation and/or surface cleanliness for eliminating delamination and bond failures, improving wire bond strength, ensuring void free underfilling of chips on circuit boards, removing oxides, enhancing die attach, and improving adhesion for die encapsulation. Typically, substrates are placed in the plasma processing system and at least one surface of each substrate is exposed to the plasma. The substrate's outermost atomic layers may be removed from the surface by physical sputtering, chemically-assisted sputtering, chemical reactions promoted by reactive plasma species, and combinations of these mechanisms. The physical or chemical action may also be used to condition the surface to improve properties such as adhesion or to clean undesired contaminants from the substrate surface.
During semiconductor manufacture, semiconductor die are commonly electrically coupled by wire bonds with leads on a metal carrier, such as a lead frame. Lead frames generally include a number of pads each having exposed leads used to electrically couple a single semiconductor die with a circuit board. One semiconductor die is attached to each pad and external electrical contacts of the die are wire bonded with nearby portions of the leads. Each semiconductor die and its wire bonds are encapsulated inside a package consisting of a molded polymer body designed to protect the semiconductor die and wire bonds from the adverse environment encountered during handling, storage and manufacturing processes as well as to dissipate the heat generated from the semiconductor die during operation. The molded packages project as three-dimensional features from one side of the otherwise generally-planar lead frame
During the molding process, the lead frame and the multiple attached semiconductor die are positioned between two mold halves. One mold half includes numerous concavities each receiving one of the semiconductor die and mimicking the shape and arrangement of the packages. The mold halves are pressed together in an attempt to seal the entrance mouths to the concavities. The molding material, which is injected into the mold, fills the open space inside the concavities for encapsulating the semiconductor die and wire bonds. However, molding material can seep out of the concavities and flow between the mold halves to form thin layers or flash on the exposed portions of the leads. This thin flash has a thickness typically less than about 10 microns. Flash affects the ability to establish high quality electrical connections with the exposed portions of the leads and, hence, with the encapsulated semiconductor die.
Various conventional approaches have been developed for alleviating the effects of flash. Flash may be prevented by covering the backside of the lead frame with tape during the molding process. However, adhesive may be transferred from the tape to the lead frame backside and remain as a residue after the tape is removed. In addition, tapes suitable for this application are relatively expensive, which needlessly contributes to the cost of manufacture. Flash may be removed after molding by mechanical and chemical techniques, or with a laser. These removal approaches also suffer from restrictions on their use. For example, the lead frame is susceptible to damage from mechanical flash removal techniques, such as chemical mechanical polishing. Chemical processes may be ineffective unless highly corrosive chemicals are used, which potentially raises issues of worker safety and waste disposal of the spent corrosive chemicals. Laser removal is expensive due to the equipment costs and leaves a residual carbon residue behind on the lead frame.
There is thus a need for a plasma processing system that can efficiently and effectively remove extraneous amounts of material, such as excess molding material, from an area on a substrate while shielding other areas on the substrate from the plasma.